toen, als or wanneer: saying 'when' in Dutch

English uses one word, when, but Dutch splits it into three: toen, als and wanneer. Which one you pick depends on the time you are talking about and on whether something happened once or again and again: Toen ik klein was, woonde ik in Rotterdam. (When I was little, I lived in Rotterdam.) Als het regent, blijf ik thuis. (When it rains, I stay home.)

Use toen for a single event in the past, als for the present or future and for a repeated event in the past, and wanneer for direct and indirect questions (and, more formally, as a synonym of als).

• toen — one thing that happened at one point in the past. It always goes with a past tense, never the present: Toen ik wakker werd, scheen de zon. (When I woke up, the sun was shining.) Toen ze de deur opendeed, stond er niemand. (When she opened the door, no one was there.)
• als for now or later — a present or future situation, often close to English if: Als het morgen mooi weer is, gaan we fietsen. (If/when the weather is nice tomorrow, we will cycle.)
• als for the repeated past — something that happened over and over. You can add telkens (each time) to make this clear: (Telkens) als hij ziek werd, ging hij naar huis. (Each time he got ill, he went home.)
• wanneer to ask when? — in a direct question (Wanneer kom je terug? — When are you coming back?) and in an indirect one (Ik weet niet wanneer de winkel opengaat. — I don't know when the shop opens.)
• wanneer as a formal als — in writing it can replace als for the present, future or repeated past: Wanneer u klaar bent, kunt u vertrekken. (When you are ready, you may leave.)

Toen and als are subordinating conjunctions, so the verb in their clause moves to the end: Toen ik thuiskwam..., Als het regent... When that clause comes first in the sentence, the main clause that follows starts with its verb: Toen ik thuiskwam, was het al donker. (When I got home, it was already dark.) This is the same verb-to-the-end pattern you get after omdat or dat. Wanneer sends the verb to the end only in an indirect question or in its formal-als use: Ik weet niet wanneer de bus vertrekt. (I don't know when the bus leaves.) In a direct question, though, wanneer keeps the verb in second position, right after it: Wanneer begint de les? (When does the lesson start?)

The most common error is using als for a single past event. English when covers everything, so learners write Als ik klein was... for a one-time memory. For one finished moment in the past, Dutch needs toen: Toen ik klein was... Keep als + past for things that happened repeatedly.

A second trap: toen never takes the present tense. Toen ik in Amsterdam woon is wrong; it must be Toen ik in Amsterdam woonde (When I lived in Amsterdam). Note also that toen has a second job as an adverb meaning then: Eerst aten we, en toen gingen we naar huis. (First we ate, and then we went home.)